Hitherto, many processes have been proposed for removing sulfur dioxide from an exhaust gas by using an aqueous solution of an alkali carboxylate as the absorbent for sulfur dioxide. The term "alkali carboxylate" as used herein includes alkaline metal carboxylates, alkali earth metal carboxylates and also ammonium carboxylate.
The reason why such a process has been proposed frequently is thought to be that sodium-, ammonium-, magnesium- and calcium carboxylate are water-soluble and have a favorable buffer capacity in the region of pH of 2 to 7 and accordingly, they act effectively in absorption of gaseous sulfur dioxide.
The processes for removing sulfur dioxide from an exhaust gas using an aqueous solution of an alkali carboxylate are classified as follows according to their by-products: (1) those in which SO.sub.2 is recovered, for instance, by using sodium citrate as the alkali carboxylate (refer to U.S. Pat. Nos. 3,970,744 and 2,142,987), (2) those in which elementary sulfur is recovered, for instance, by using sodium citrate (refer to U.S. Pat. Nos. 4,083,944 and 3,933,994) or by using sodium sulfosuccinate (refer to U.S. Pat. No. 2,729,543), (3) those in which calcium sulfite is by-product (refer to Japanese Patent Application Laying-Open No. 55-61923/80) and (4) those in which gypsum is by-product (refer to Japanese Patent Application Laying Open No. 49-104881/74).
However, the above-mentioned processes are disadvantageous due to the gradual consumption of the carboxylate by decomposition during the operation of the process of desulfurization.
For example, in the step wherein SO.sub.3.sup.2- and HSO.sub.3.sup.- which are formed from sulfur dioxide absorbed into the aqueous solution containing the alkali carboxylate are oxidized by oxygen, an active radical is formed as an intermediate reaction product and the radical induces an oxidative decomposition of the carboxylate ion which co-exists with the radical. The amount of carboxylate ion decomposed is nearly proportional to the sum of amounts of SO.sub.3.sup.2- and HSO.sub.3.sup.- which are oxidized by oxygen.
Accordingly, in each of the above-mentioned processes (1), (2) and (3), since oxidation in the absorption tower is carried out only by the oxygen contained in the combustion exhaust gas, only a small part of SO.sub.3.sup.2- and HSO.sub.3.sup.- is oxidized and the amount of oxidatively decomposed carboxylate ion is also small enough so as to render the problem of consumption by decomposition of the carboxylate ion negligible. However, in the process of type (4) wherein sulfur dioxide is recovered as gypsum after forcibly oxidizing the whole amount of absorbed sulfur dioxide, the decomposition and elimination of the carboxylate ion becomes a significant problem which is not negligible.
As a method for suppressing the decomposition of carboxylate ion in the process of absorbing sulfur dioxide contained in the combustion exhaust gas by the use of alkali carboxylate to obtain gypsum as a by-product, a method of having manganese ion or copper ion co-exist with the alkali carboxylate in the aqueous absorbent has been known (refer to Japanese Patent Publication No. 53-42317/78). According to this known method, in the case where divalent manganese ion or copper ion is present at a concentration of 100 ppm, the amount of gaseous carbon dioxide generated by the decomposition of the carboxylate ion is about half when compared to the case where such a metal ion is absent. In the case where divalent manganese ion or copper ion is present at a concentration of 3000 ppm, the amount of gaseous carbon dioxide by the decomposition of carboxylate ion is less than one tenth as compared to the case where no metal ion is present.
However, in the case where the aqueous absorbent is neutral to alkaline, the manganese ion or the copper ion is converted to its hydroxide which is insoluble in the aqueous solution. Thus in order to prevent such a phenomenon, it is necessary to maintain the aqueous absorbent acidic.
On the other hand, in the case where an aqueous solution of an alkali salt of a volatile carboxylic acid is used as the aqueous absorbent, a free carboxylic acid is formed upon the absorption of gaseous sulfur dioxide and a part of the free carboxylic acid thus formed is dissipated with the treated exhaust gas when the pH of the aqueous absorbent which is brought into contact with the exhaust gas is at a low level.
Such a dissipation of carboxylic acid may be prevented by washing the de-sulfurized exhaust gas with an alkaline solution. However, this additional washing of the de-sulfurized exhaust gas makes the process more complicated. Moreover, in the case where an aqueous absorbent at a high pH level is used for preventing the dissipation of the carboxylic acid, manganese ion or copper ion which suppress the decomposition of the carboxylate ion cannot co-exist with the carboxylate ion in the aqueous absorbent.
As has been described, a process for removing sulfur dioxide from a combustion exhaust gas, by which the dissipation of carboxylic acid and also the decomposition of the carboxylate ion are prevented, has not been known.
Since in a process for removing sulfur dioxide from a combustion exhaust gas, which is of the type of by-producing gypsum while using an alkali carboxylate, the amount of the carboxylate ion decomposed is nearly proportional to the sum of amounts of SO.sub.3.sup.2- and HSO.sub.3.sup.- oxidized and also depends on the concentration of the carboxylate ion, the concentration of the alkali carboxylate in the aqueous absorbent is preferably as low as possible within the limit which is necessary for the absorption of gaseous sulfur dioxide. As a result of the inventors' studies on the decomposition of carboxylate ion, it has been found that the decomposition of carboxylate ion depends largely on the concentration of chloride ion in co-existence, and in the case where its concentration is higher than 0.4% by weight, the amount of carboxylate ion decomposed shows reduction and in the case of higher than 2% by weight, the decomposition is remarkably suppressed. The present invention depends on the above-mentioned findings.